A number of such devices for manual operation have been developed in recent years, these being designed to avoid any need for complex procedures, and thus be suitable for use by lay persons. For example, to avoid the necessity of a timed sequence of reagent additions to an analyte, devices have been developed to automatically sequentially deliver such reagents by use of multiple capillary flow channels. Examples of these capillary flow diagnostic devices are further described in Patent Specification WO90/11519 and in co-pending UK Patent Applications GB-2261283 and GB-2261284.
In some applications of devices of this type a user is looking for a simple colour change to confirm the presence of a specific analyte in a sample. In others, the user may be seeking a quantified result such as a certain degree of colour change, and it is in these latter applications that a need arises to accurately measure out, or define, a desired volume of the sample onto the device.
Currently, the sample volume is measured out and applied to an analytical site of the diagnostic device, the analytical site comprising a quantity of antibodies immobilised within a specific region. The volume measuring is done using a hand pipette or capillary tube. Pipettes are expensive precision instruments and considerable skill is needed to achieve accurate results. Capillary tubes are less expensive, and may include a porous plug to define the sample volume. However, they are usually made of glass and therefore readily breakable in mass usage, and in any case an inexperienced user can find them difficult to use.
If an undefined amount of sample is applied to an analytical site, then there may well be an `error volume` of sample deposited beyond the boundaries of the immobilised region. This is in general terms unlikely to create a major inaccuracy with respect to sample lying downstream or laterally of the immobilised region, but error sample lying upstream of the immobilised region will be passed by liquid flow through the region and there may be reaction between the antigen of interest contained therein and the antibodies at the analytical site, thus ultimately leading to an inaccurate result.
One known method of sample volume definition is to incorporate a manually operated valve mechanism, which shears off a defined volume of sample in;to the diagnostic device. Such a device is described in M. P. Allen et al, Clinical Chemistry 36 (1990) p.1591-1597. The measuring out of sample volume is thus automatically realised and possibilities for error are thus greatly reduced. However, the mechanism involves precision moving parts and is thus relatively expensive to manufacture.